The present invention relates to rubber-reinforcing coated glass fibers, which have been subjected to a surface treatment for improving bonding between the glass fibers and a parent rubber to be reinforced. It relates particularly to rubber-reinforcing coated glass fibers to be embedded in a parent rubber (e.g., hydrogenated nitrile rubber) to improve tensile strength and dimensional stability of the parent rubber for the use as a mechanical belt such as automotive timing belt.
It is generally widely conducted to embed high-strength fibers (e.g., glass fibers, nylon fibers, and polyester fibers) as a reinforcing member in a parent rubber (e.g., mechanical belt and tire) in order to improve strength of the parent rubber. The reinforcing fibers to be embedded in the parent rubber are required to have a good adhesion to the parent rubber to prevent exfoliation at their interface. Glass fibers themselves are inferior in adhesion to rubber and therefore cannot serve as a reinforcing member.
In order to improve adhesion between parent rubber and glass fibers and to prevent exfoliation at their interface, coated glass fibers are embedded in mechanical belt. The coated glass fibers may be prepared by the steps of (a) twining glass filaments into glass fibers (yarns); (b) dispersing a resorcinol-formaldehyde resin and a latex in water to prepare a coating solution; (c) applying the coating solution to the glass fibers to form a precursory layer thereon; and (d) drying the precursory layer into a coating layer, thereby preparing the coated glass fibers. Under a high temperature environment, the coated glass fibers are embedded in a parent rubber, and the fiber-reinforced rubber is formed into a mechanical belt. The above-prepared coated glass fibers are not necessarily sufficient in adhesion strength. For example, automotive timing belt is used under a high temperature environment in an engine compartment. Therefore, a heat resistant rubber (e.g., a hydrogenated nitrile rubber cross-linked using sulfur or peroxide) is used as the parent rubber. When a timing belt containing the above-prepared coated glass fibers is continuously bent under high temperature, there may occur an exfoliation at an interface between the parent rubber and the coated glass fibers in use for a long time.
In order to provide mechanical belts having reliability for a long time under high temperature environment due to good adhesion between hydrogenated nitrile rubber and glass fibers, there are proposals for coating glass fibers with a first coating layer (prepared from the above coating solution in which a resorcinol-formaldehyde resin and a latex are dispersed in water) and a second coating layer of a composition different from that of the first coating layer.
Japanese Patent Laid-open Publication JP-A-63-126975, corresponding to Japanese Patent Examined Publication JP-B-2-4715, discloses a coating solution containing a halogen-containing polymer and an isocyanate for forming the second coating layer on the first coating layer. It is disclosed therein that the coated glass fibers are embedded in a hydrogenated nitrile rubber.
Japanese Patent Laid-open Publication JP-A-11-241275, corresponding to Japanese Patent 3201330, discloses (a) a first coating solution containing a resorcinol-formaldehyde condensate and a rubber latex for forming the first coating layer on glass fibers and (b) a second coating solution containing a rubber-blended material, a vulcanizing agent, and a maleimide-based vulcanization accelerator for forming the second coating layer on the first coating layer. It is disclosed therein that the coated glass fibers are embedded in a mixed rubber of a hydrogenated nitrile rubber and a hydrogenated nitrile rubber in which zinc methacrylate is dispersed.
Conventional rubber-reinforcing glass fibers are, however, still not sufficient in adhesion to heat resistant rubber (e.g., a hydrogenated nitrile rubber cross-linked using sulfur or peroxide). In fact, adhesion strength may become inferior in use for a long time, and exfoliation may occur at their interface. With this, fiber-reinforced rubbers may fail to maintain the original strength and may become inferior in dimensional stability. Thus, there is a demand for improve adhesion strength between glass fibers and heat resistant rubber as the parent rubber.